The invention relates in general to a method and apparatus for stacking sheets in a printing press. More specifically, after the run through the printing module of a printing press, the printed sheets are stacked in a stacking unit of the printing press where various solutions are used to pick up the sheet from the sheet path and to deposit it on the stacking unit.
One prior art mechanism for stacking sheets involves the gripping of the sheet in a transport path with a mechanical gripper, which has two gripper plates that partially grasp the sheet and carry it from the path to a stacking unit. The gripper then deposits the sheet on the stacking unit. However, the speed of this stacking solution is limited, since only one sheet per run is grasped by the gripper and carried to the stacking unit. The speed characteristic is an important criterion for modern high speed, high productivity printing machines.
This invention deposits sheets on a stacking unit quickly, cost effectively and reliably. The invention is directed to a method for depositing sheets, and a stacking device that contains two gripping devices, for alternately picking up and depositing sheets relative to a stack. A first gripping device picks up a sheet advantageously within reach of the sheet transport, while a second gripping device simultaneously holds the sheet stack, whereby the sheets to be deposited are at a lower level to prevent the sheet stack from moving during this process. The arrangement of the sheet on this sheet stack is in registered alignment, or stacking may also be offset lengthwise to the sheet transport path. Conveyor rollers are used for this purpose, which slow down the run of the transported sheet at a specific point in time and thus determine the stacking of various sheets in an offset manner in the lengthwise direction. The sheets may then be removed by the operator in stacks from the printing press at the rate of approximately one offset stack per printing job.
The invention and its advantages will be better understood from the ensuing detailed description of preferred embodiments, reference being made to the accompanying drawings in which like reference characters denote like parts.